A sheet-fed printing machine of such type is disclosed in U.S. Pat. No. 2,138,405. The chain-guided gripper carriages provided to transport the sheet through the printing machine in a single gripper closure are intermittently locked to the impression cylinders of the printing units in order to synchronize the circumferential speeds of the cylinder surfaces and of the chains and of the guide rails connected to the latter. This is one of the most important requirements for good register for perfect multi-colour printing.
To obtain good register, very accurate operation also is required in the reproduction, stage through assembly up to the plate copy. In some cases, special register systems are used and special coloured assembly sheets are copied for assembly in order to facilitate the process. Setting up a printing job, however, largely comprises bringing the various printing plates into register with one another on the printed sheet. To obtain good register, therefore, the printing plates must be carefully clamped, for which purpose the plates bearing the test for printing are immovably clamped on the cylinders which carry said plates in order to align them so that they are in register. This, however, is a very time-consuming and difficult operation particularly if the printing plate is clamped in a skewed position, i.e., with the start of the print not parallel to the axis of the plate cylinder, or if any adjustment errors or inaccuracies which may occur at the reproduction stage from assembly up to the plate copy require compensation.
Irregularities occur in sheet-fed printing machines because of elongation of the gripper carrying chains during operation. For example, stretching and elongation or contraction of the chains cause difficulties resulting in disturbances to the effective operation between the gripper on the chains and the grippers from which the sheets are received, such as a feed drum. Radial or vertical positioning of the grippers relative to the direction of their travel through the machine may also be disturbed due to uneven chain lengths. Dynamic loading occurs due to speed changes in the chains which are guided in wave-like fashion through the sheet-fed printing machine, the gripper carriages which are suspended therefrom being guided over parabolic guide rails within and outside the cutout of each impression cylinder, and such dynamic loading has an adverse effect on register. This particularly occurs because non-adjustable mechanical synchronization means are provided in the form of positive fixing devices which close automatically during operation, and this results in deterioration of synchronization of the circumferential speeds of the surfaces of the impression cylinders and of the chain-guided gripper carriages or the parabolic or like guide rails coupled thereto.
The textbook "Einfuhrung in den Offsetdruck", by Wolfgang Walenski, Eggen-Fachbuchreihe, 1975, page 113, shows a sheet-fed printing machine having a double-size impression cylinder and double-size take-off and transfer drums with correspondingly large sprocket wheels, in which it is known that the transfer of the sheets from the first printing unit to the second can be carried out by chain-guided gripper systems using two special locking systems disposed diametrically opposite one another on the double-size drums, the gripper systems being adjustable both radially and tangentially for exact transfer. The special locking system is controlled intermittently so that the sheets are transferred to the double-size take-off drum or are transferred from the latter to the grippers of the delivery unit only when the printing is completed, even in the case of a maximum format. It is a disadvantage that two sheet transfers occur between the printing units, and register differences may occur due to transfer difficulties.